In a conventional typical steam turbine plant, steam from a boiler is passed through a steam valve device and is then fed to a steam turbine. The steam after working in the steam turbine is condensed into water by a steam condenser, and the water is pressurized by a feed pump to be fed again to the boiler. In this manner, the steam circulates in the steam turbine plant.
The steam valve device has a main steam stop valve and a steam regulating valve disposed on the downstream side of the main steam stop valve. The main steam stop valve can stop steam flowing in a steam turbine in a moment when an emergency occurs in the turbine. The steam regulating valve controls the flow rate of steam fed to the steam turbine.
In some steam valve device, the main steam stop valve and steam regulating valve are integrated. Various integration approaches have been proposed and, for example, there is known a configuration in which the main steam stop valve and a steam regulating valve are integrated through an intermediate flow pipe and they are each driven by an oil cylinder disposed above a casing.
In the abovementioned steam valve device, one main steam stop valve and one steam regulating valve constitute one pair. Thus, the steam regulating valve in a conventional steam turbine plant is suitable for a steam turbine plant adopting a throttle control system. In the throttle control system, the steam regulating valve is not fully open in a partial load condition of the steam turbine, incurring a throttle loss.
Meanwhile, steam turbine plants are being increased in capacity of the single power output (generation capacity) and, correspondingly, the diameter of each valve of the steam valve device tends to increase. Under such circumstances, an increase in the efficiency of the steam turbine in its partial load condition is required. For such a steam turbine plant focusing on the efficiency in the partial load condition of the steam turbine, a nozzle control system is suitable. In the nozzle control system, parts of the steam regulating valves are substantially fully opened at the partial load condition of the steam turbine, suppressing the throttle loss.
In a steam turbine plant adopting the nozzle control system, a configuration in which a nozzle box serving as a member for feeding steam to a turbine stage of the steam turbine is partitioned into a plurality of sections in the circumferential direction is used. In the case where the above steam valve device is applied to a steam turbine plant adopting the nozzle control system, it is necessary to provide the number of valves corresponding to the number of the circumferentially-arranged sections of the nozzle box. For example, in the case where the nozzle box is partitioned into four sections in the circumferential direction, four pairs of valves, i.e., four main steam stop valves and four steam regulating valves are required. Thus, manufacturing cost is increased.